1. Technical Field
The present invention relates to an electromechanical generator for converting mechanical vibrational energy into electrical energy. In particular, the present invention relates to such a device which is a miniature generator capable of converting ambient vibration energy into electrical energy for use, for example, in powering intelligent sensor systems. Such a system can be used in many areas where there is an economical or operational advantage in the elimination of power cables or batteries.
2. Description of the Prior Art
There is currently an increasing level of research activity in the area of alternative power sources for wireless sensors, such devices being described in the art as being used for ‘energy harvesting’.
It is known to use an electromechanical generator for harvesting useful electrical power from ambient vibrations. A typical magnet-coil generator consists of a spring-mass combination attached to a magnet or coil in such a manner that when the system vibrates, a coil cuts through the flux formed by a magnetic core. The mass which is moved when vibrated is mounted on a cantilever beam. The beam can either be connected to the magnetic core, with the coil fixed relative to an enclosure for the device, or vice versa.
The use of a cantilever beam as the spring results in a number of technical problems. First the motion of a cantilever beam about its central position is not linear. Therefore, any magnetic coupling between the magnet and the coil is not optimized unless these components are relatively complex in form.
In addition, either the coil or the magnet may be mounted for vibrational resonant motion on the cantilever beam. In either case, it is necessary for the mass that is mounted on the cantilever beam to be carefully controlled in order to ensure that the natural frequency corresponds to the designed natural frequency. For some constructions, such careful control of the mass may be difficult to achieve in practice, and can also greatly increases manufacturing costs, because of the difficulties and complications of producing a mass of the desired amount.
Furthermore, it is generally known in the art that as a rule the greater the mass of the spring-mass combination of the magnetic core generator, the greater the output electrical power. However, in practice there is difficulty loading excess mass onto a cantilever beam, while retaining not only careful control of the natural frequency but also a compact and robust structure.
The primary problem however of known electromechanical generators, in particular those incorporating cantilever beams, is that the electrical output is generally too low. This is a factor of the spring-mass combination of the magnetic-coil generator being such that only a narrow resonant bandwidth is provided around a nominal natural frequency. Accordingly, since in practice the vibration tends often to be within an even broader frequency band than the bandwidth of the device about the natural frequency, in practice the magnetic coupling is reduced as compared to the theoretical design maximum. This in turn means that the maximum voltage which can reliably be extracted from the electromechanical generator is rather low. This can, in turn, seriously restrict the operational applications of the electromechanical generator.
The two main problems of known electromechanical generators are that they produce too little electrical power output and have too narrow an operating bandwidth. Power output can be improved by increasing the moving mass within the device, but bandwidth can only be improved by increasing the electromagnetic coupling. Higher electromagnetic coupling allows more power to be extracted when the device is not operating at exactly its natural frequency.
In a paper entitled “Self-Powered Signal Processing using Vibration-based Power Generation” Amir Therarja et al, IEEE Journal of Solid-State Circuits, Vol. 33, No. 5, May 1998 the feasibility of operating a digital system from power generated by vibrations in its environment, in particular by using a moving coil electromagnetic transducer as the power generator, was explored. In laboratory experiments, an inertial electromechanical generator was proposed and a prototype made consisting of a mass connected to a helical spring with the other end of the spring being attached to a rigid housing so that mass depended downwardly on the helical spring from the rigid housing. As the housing was vibrated, the mass moved relative to the housing and energy was stored in the mass-spring system. A wire coil was attached to the mass and moved through the field of a permanent magnet B located beneath the coil as the mass vibrated. The moving coil cut a varying amount of magnetic flux and in turn induced a voltage on the coil in accordance with Faraday's law. This voltage was the electrical output of the generator and was the input of a voltage regulator.
This laboratory based model would have no utility for practical applications in the field because it would not be sufficient robust and would be, unlike the use of a cantilever beam, subject to interference by lateral vibration.
In a paper entitled “AA Sized Micro Power Conversion Cell for Wireless Applications” by Yuen et al, published in the Proceedings of the Fifth World Congress on Intelligent Control and Automation, Jun. 15-19, 2004, Hangshu, People's Republic of China, there is disclosed a prototype micropower generator having the same size and dimensions of an AA size battery. This comprised a laser-micro machine resonating spring which was spiral and planar in structure and was attached to a rare earth permanent magnet. This was mounted in an inner housing. The outer surface of the inner housing was surrounded by a coil. The magnet could vibrate up and down within a cylindrical chamber defined in the inner housing.
This device suffers from the problem that very high amplitude of vibration, is required to obtain useful power output; 250 micron vibration amplitude at 100 Hz which equates to around 10 g of acceleration amplitude (where g is the acceleration due to gravity). Also, the fabrication of the micro-machined spiral spring is difficult and expensive and the paper concludes that if a spring could be designed to vibrate in a horizontal plane with rotation, rather than to vibrate in a vertical direction relative to the coil, the voltage output could be increased and the stress on the spring could be reduced.
In a paper entitled “Architecture for vibration-driven micropower generators”, by Aitcheson et al, published in the Journal of Micromechanical Systems, Vol. 13, No. 3, June 2004, pp. 335-342, various electromechanical generators are disclosed. In particular, a velocity-damped resonant generator (VDRG) is disclosed which consists of a damper for extracting energy from a mass-spring system. Such a damper may consist, for example, of a magnet-coil generator, such as the combination of two magnets mounted on a keeper to form a C-shaped core with a coil placed in the air-gap between the magnets at right angles to the direction of movement of the mass on a cantilever beam.
While these prior disclosures all produce a useful mechanism for designing a theoretical electromechanical generator, when an electromechanical generator is used in a practical application, it is not possible accurately to predict the natural frequency or the optimal damping factor. The electromechanical generator is designed and set up for what is believed to be the likely operating conditions. However, there is no guarantee that the practical operating conditions correspond to the theoretical ideal used to set up the electromechanical generator for the specific application. In practice, an electromechanical generator is set up to be operable across a narrow range of likely operating conditions, in particular with the damping factor being set up so that the power output is within a range encompassing the optimal power output. However, it is very unlikely that the actual power output is optimized for the specific application. Consequently, the electromechanical generator would not operate at maximum efficiency of the conversion of mechanical vibration energy into electrical energy, and thereby into useful electrical power.
Also, the frequency of ambient vibration may change during operation. The known electromechanical generator may not be able to operate at maximum efficiency as a result of such a change.
Yet further, the damper of the electromechanical generator incorporates a sprung mass that oscillates about a central position at a frequency intended to correspond to the frequency to which the device is to be subjected in use. The amplitude of the resonant vibration depends upon a number of variables, in particular the frequency and magnitude of the driving vibration, the Q-factor of the resonator, the resonator mass and its natural frequency.
These variables are not all predictable from the actual conditions encountered when the electromechanical generator is put into use in the field to harvest energy from a vibrating body. The amplitude of vibration of the sprung mass may vary with time, in an intermittent and unpredictable manner.